1. Technical Field
This invention relates to a rotational speed detector for an internal combustion engine.
2. Description of the Relevant Art
The rotational speed of an internal combustion engine for an engine driven heat pump should be controlled accurately and properly to conform with a required rotational speed because the rotational speed is directly related to an air conditioning maintained by the engine-driven heat pump.
Further, if the rotational speed of the engine repeatedly rises and falls, the noise of the engine may be uncomfortable or disturbing.
Furthermore, when the rotational speed of the engine is reduced, the engine may stop due to the load of the heat pump unless the rotational speed of the engine is detected accurately and so controlled.
It is well known in the art to control the amount of the fuel supplied to the engine in response to a difference between an actual rotational speed and the required speed by opening or closing a throttle valve so as to control the rotational speed of the engine.
Under such control, it is required for a detector to detect the actual rotational speed accurately.
Meanwhile, a conventional ignition system comprises an ignition coil which has a primary winding and a secondary winding and supplies high voltage to a spark plug, a permanent magnet which is rotated by the engine, and a switching device which is connected to the primary winding of the ignition coil. When the switching device turns on, magnetic energy charges the ignition coil, and then, when the switching device turns off, the charged magnetic energy is discharged toward the spark plug as a high voltage impulse.
In the conventional device, the rotational speed of the engine is detected by measuring a period of time between the separate events when the switching device repeatedly turns on. Accordingly, an ignition signal between the switching device and the ignition coil is detected in order to measure this period and, consequently, the rotational speed.
Although a D.C. voltage supplied to the ignition coil has a low value of about 12 volts, the detected ignition signal at the point between the switching device and the ignition coil includes a very high voltage impulse of about 270 volts and of high frequency (short duration) as shown in FIG. 6.
Further, this detected voltage sometimes includes an unexpected pulse F.sub.E of longer duration than the high voltage impulse and of low frequency caused, for example, by a vibration of the ignition coil as shown in FIG. 7.
Although the effects of high frequency impulses may be reduced by means of a conventional low pass filter, the unexpected pulse F.sub.E will not reduced by the low pass filter, because the unexpected pulse F.sub.E includes a low frequency signal passed by the filter. Further, this unexpected pulse F.sub.E interferes in the detection of the actual rotational speed, because this unexpected pulse F.sub.E bears a close relation to the rotational speed of the engine.
Accordingly, the conventional device sometimes fails to detect a true rotational speed by acting on the unexpected pulse F.sub.E.